The suspected gunman who terrorized an Atlanta-area elementary school, firing in the front office and at officers, was armed with an assault rifle and nearly 500 rounds of ammunition, police said today.

"He walked in with 498 rounds of ammunition. Fortunately, this came to an end quietly, without incident," Alexander said at a news conference. "I think we can all make a reasonable assumption he came here to do some harm."

Det. Ray Davis said an AK-47-type assault rifle was the only weapon recovered from the scene.

The weapon has been taken to an Alcohol, Tobacco and Firearms laboratory for testing, Davis said, and he said police believe the suspect obtained the weapon from an acquaintance, whom they are working to locate.

However, a source with knowledge of the case told ABC News the chain of custody for the weapon includes "multiple" people, and not just the acquaintance mentioned at the news conference.

Meanwhile, Hill's public defender waived his first court appearance today and no bond was set.

Police said Hill will face charges including aggravated assault on a police officer, terroristic threats and possession of a firearm by a convicted felon.

School Bookkeeper Kept Calm on 911 Tapes

Antoinette Tuff, the school bookkeeper who is credited with persuading the suspect to put his weapons down and surrender to police, was hailed as a hero today by Alexander.

"She was able to talk [the suspect] down," Alexander said. "Had that not been the case, this certainly could turned into something really ugly."

In newly released 911 audio, Tuff can be heard reasoning with the suspect and letting him know that everything was going to be fine.

"I can help you. You want me to talk to them? And try?" she asked. "Well, let me talk to them and try to work it out so that you don't have to go away with them for a long time."

Hill, according to Tuff, said he had no reason to live because nobody loved him.

"I just explained to him that I loved him," Tuff told ABC News in an exclusive interview Tuesday night. "I didn't know much about him. I didn't know his name but I did love him and it was scary because I knew at that moment he was ready to take my life along with his, and if I didn't say the right thing, then we all would be dead."

Hill also allegedly ordered staffers to call a local television channel, ABC affiliate WSB-TV, to request that a camera crew record him "killing police."

"He said that he didn't have any reason to live and that he was going to die today," Tuff said. "He was going to end his life and take all of the cops and everybody with him."

The final moments of his surrender, when Tuff persuaded him to lay out his weapons and get on the floor, can be heard on the 911 tape.

"It's going to be OK, sweetheart," she said. "I just want you to know that I love you and I am proud of you. That's a good thing that you are giving up. ... We all go through something in life."

Police, including U.S. marshals, entered the school and found the suspect in the office, where authorities said the standoff ended without incident.

No one was injured and Hill did not make it past the main office inside the school, police said.

Alexander praised the multiple agencies who worked to end the standoff, saying it could have easily turned into "another Sandy Hook," referring to the elementary school massacre in December 2012.

Suspect's Family Says He Has History of Mental Disorders

While police have declined to comment on a possible motive, the suspect's brother, Timothy Hill, said his brother has a "long history of medical disorders," and it was only a matter of time before he was bound to "do something stupid."

"I honestly can tell you he has got a long history of medical disorders, including bipolar, and that could make you snap on a dime," Timothy Hill told ABC News. "My mom's almost looked like a drugstore at one point. There was so many different medications he was on."

Hill, 22, said he's not close to his brother and believed he last saw him in January 2011. Hill did not disclose his brother's complete mental health history but said he was taking drugs for treating attention-deficit hyperactivity disorder as early as age 6.

Hill said his brother had once threatened to kill him.

"I had a feeling he was going to, eventually, one day, do something stupid, but not of this magnitude," he said.

Natasha Knotts told the Associated Press that Michael Hill lived with her and her husband for a time when he was in his late teens after he started coming to their church.

She said Hill called her sister before the shooting and told her he had a rifle, but did not say what he was planning to do. Knotts said she believed his alleged actions on Tuesday were a cry for help.

"This is something that's totally out of his character. This is not him. This is not the Mike that I know," she said. "For anyone that knew Mike, this was a total devastation."

Police Probing How Suspect Entered School

Authorities believe Hill might have entered the school by closely following a person authorized to enter the building. But once inside, he only went as far as the front office.

"Once we found out where he was located inside the school, we actually gained entry through a side building in the school," DeKalb County Police Det. Clay Hobbs said today on "Good Morning America." "Luckily, we had an officer who was familiar with the layout of the school and made it directly to the office where Antoinette had the man already on the ground, so we could apprehend him and take him into custody."

SWAT teams were sent classroom to classroom to evacuate students, some as young as pre-kindergarten.
Authorities have yet to establish a motive or determine whether the suspect had a link to the school.

The shooting came on the second week of classes at the charter school. Classes for the elementary students will be held at McNair High School today.

After her ordeal, Tuff said she would be back to work today.

"Yes, I will be back," she said, "sitting in that same seat, blessing that next person."